1. Technical Field
This invention relates to processes for preparing ultra-high molecular weight polyethylene (“UHMW PE”) filaments and multi-filament yarns, and articles produced therefrom.
2. Description of the Related Art
Ultra-high molecular weight poly(alpha-olefin) multi-filament yarns have been produced possessing high tensile properties such as tenacity, tensile modulus and energy-to-break. The yarns are useful in applications requiring impact absorption and ballistic resistance such as body armor, helmets, breast plates, helicopter seats, spall shields, composite sports equipment such as kayaks, canoes bicycles and boats; and in fishing line, sails, ropes, sutures and fabrics.
Ultra-high molecular weight poly(alpha-olefins) include polyethylene, polypropylene, poly(butene-1), poly(4-methyl-pentene-1), their copolymers, blends and adducts having a molecular weight of at least about 300,000 g/mol. Many different techniques are known for the fabrication of high tenacity filaments and fibers formed from these polymers. High tenacity polyethylene fibers may be made by spinning a solution containing ultra-high molecular weight polyethylene. Ultra-high molecular weight polyethylene particles are mixed with a suitable solvent, whereby the particles are swelled with and dissolved by the solvent to form a solution. The solution is then extruded through a spinneret to form solution filaments, followed by cooling the solution filaments to a gel state to form gel filaments, then removing the spinning solvent to form solvent-free filaments. One or more of the solution filaments, the gel filaments and the solvent-free filaments are stretched or drawn to a highly oriented state in one or more stages. In general, such filaments are known as “gel-spun” polyethylene filaments. The gel spinning process is desirable because it discourages the formation of folded chain molecular structures and favors formation of extended chain structures that more efficiently transmit tensile loads. Gel-spun filaments also tend to have melting points higher than the melting point of the polymer from which they were formed. For example, high molecular weight polyethylene having a molecular weight of about 150,000 to about two million generally have melting points in the bulk polymer of 138° C. Highly oriented polyethylene filaments made of these materials have melting points of from about 7° C. to about 13° C. higher. This slight increase in melting point reflects the crystalline perfection and higher crystalline orientation of the filaments as compared to the bulk polymer. Multi-filament gel spun ultra-high molecular weight polyethylene (UHMW PE) yarns are produced, for example, by Honeywell International Inc.
Various methods for forming gel-spun polyethylene filaments have been described, for example, in U.S. Pat. Nos. 4,413,110; 4,536,536; 4,551,296; 4,663,101; 5,032,338; 5,578,374; 5,736,244; 5,741,451; 5,958,582; 5,972,498; 6,448,359; 6,746,975; 6,969,553; 7,078,099; 7,344,668 and U.S. patent application publication 2007/0231572, all of which are incorporated herein by reference to the extent compatible herewith. For example, U.S. Pat. Nos. 4,413,110, 4,663,101 and 5,736,244 describe the formation polyethylene gel precursors and the stretching of low porosity xerogels obtained therefrom to form high tenacity, high modulus fibers. U.S. Pat. Nos. 5,578,374 and 5,741,451 describe post-stretching a polyethylene fiber which has already been oriented by drawing at a particular temperature and draw rate. U.S. Pat. No. 6,746,975 describes high tenacity, high modulus multifilament yarns formed from polyethylene solutions via extrusion through a multi-orifice spinneret into a cross-flow gas stream to form a fluid product. The fluid product is gelled, stretched and formed into a xerogel. The xerogel is then subjected to a dual stage stretch to form the desired multifilament yarns. U.S. Pat. No. 7,078,099 describes drawn, gel-spun multifilament polyethylene yarns having increased perfection of molecular structure. The yarns are produced by an improved manufacturing process and are drawn under specialized conditions to achieve multifilament yarns having a high degree of molecular and crystalline order. U.S. Pat. No. 7,344,668 describes a process for drawing essentially diluent-free gel-spun polyethylene multifilament yarns in a forced convection air oven and the drawn yarns produced thereby. The process conditions of draw ratio, stretch rate, residence time, oven length and feed speed are selected in specific relation to one another so as to achieve enhanced efficiency and productivity.
Despite the teachings of the foregoing documents, there remains a need in the art for a process for preparing high tenacity UHMW PE multi-filament yarns with greater productivity that is suitable for commercial scale manufacturing. The theoretical strength of UHMW PE yarn is around 200 g/denier based on C—C bond calculation. However, fibers of such maximum tenacity are not presently achievable due to processability limitations of the UHMW PE polymer. For example, it is understood that UHMW PE fibers having high tenacities correspond to UHMW PE starting material having high molecular weight. Accordingly, UHMW PE fiber tenacity may theoretically be increased by increasing the molecular weight of the UHMW PE raw material from which they are fabricated. However, increases in polymer molecular weight leads to various processing drawbacks. For example, fibers having high tenacities require slower and more carefully controlled fiber drawing to avoid breaking of the fiber during stretching. Such slower fiber drawing is undesirable, however, because it limits fiber output and the commercial viability of the process. Increases in polymer molecular weight also requires elevated extrusion temperatures and pressures to handle the higher molecular weight material, but these more severe conditions may accelerate polymer degradation and limit the attainable fiber tensile properties.
Due to these limitations, the manufacture of high tenacity UHMW PE yarns, particularly those having a yarn tenacity of 45 g/denier or greater, is a challenging and exceedingly slow undertaking. To be sure, any related art discussing the fabrication of UHMW PE fibers having a tenacity of 45 g/denier or more, such as U.S. Pat. No. 4,617,233, refer to achievements that are not capable of being translated to a realistic, commercially viable scale. No method of the related art is presently known that is capable of manufacturing UHMW PE yarns having a tenacity of 45 g/denier or more at a commercially viable throughput rate. Accordingly, there remains a need in the art for a more efficient process for producing strong UHMW PE yarns at high production capacity. The present invention provides a solution to this problem in the art.